The aim of this study is to understand behavioral mechanisms that regulate vertebrate social organization, in particular the agonistic and affiliational processes that control the spatial relationships of individuals. The proposed research addresses three problems related to this aim: (1) the interaction of social environment and maturation in the development of male aggression; (2) the relationships among territorial aggression, dominance, and defense of an individual distance; and (3) the importance of spatial and vocal coordination in heterosexual affiliations. Six related species in the avian genera Agelaius and Quiscalus have been chosen to allow comparisons between species with differences in territoriality and heterosexual affiliations. The study will integrate field studies and experiments with captive birds to examine the three problems just enumerated. (1) Experiments with captive male red-winged blackbirds will investigate the hormonal basis for previously established differences in first-year and older males' agonistic behavior. (2) Field studies will measure the spatial distributions of individuals' activities, in order to investigate determinants of spatial variation in aggression by territorial residents. Experiments with captive male blackbirds and finches will determine the relationship between defense of an individual distance and testicular activity in the course of the spring in a territorial and a non-territorial species. Further experiments on defense of an individual distance will investigate the effects of dominance rank, androgen treatment, familiarity with the aviary, and crowding. (3) Field studies of species with different mating systems will measure vocal and spatial coordination in their heterosexual affiliations.